1. Field of the Invention
The present invention relates to a technique for forming an electrical contact a conductor and an underlying region separated therefrom by a dielectric layer.
2. Description of the Prior Art
In the production of integrated circuits and other solid state devices, electrical contacts are required between an overlying conductive layer and an underlying region separated therefrom by a dielectric material. The overlying layer is typically a metal, and the underlying region is typically a doped semiconductor region, or another metal layer. Referring to FIG. 1, in one typical prior art technique, a silicon semiconductor material 10 has formed therein a doped region 11 which may be, for example, the source or drain of a field effect transistor. Formed thereon is dielectric layer 12, typically of a relatively low softening point (e.g., less than 1100.degree. C. in the heating ambient used) glass. This dielectric layer typically has a thickness of less than 10,000 angstroms, and presently about 7,000 angstroms in one typical process. There is then formed in layer 12 an opening, also termed in the art a "window", having a width d as indicated. The etching technique utilized to form the window frequently is anisotropic; that is, tending to remove material faster in the vertical direction as viewed as compared to the horizontal direction. This process thus forms vertical sidewalls as shown. For this purpose, reactive ion etching is frequently utilized. Since it is desired to deposit a conducting material in this window, the sidewalls typically need to be smoothed. This allows a continuous conducting layer of material to be deposited. In particular, aluminum is notorious for not forming a continuous layer when deposited over vertical sidewalls.
To provide for smoothing the sidewalls, the dielectric material of region 12 is typically a phosphosilicate glass (p-glass), a borosilicate glass, or a glass containing both phosphorous and boron. A heating step, typically 900.degree. C. to 1100.degree. C., then causes the sidewalls to flow, forming a smooth transition region 23, as indicated in FIG. 2. Referring to FIG. 3, an aluminum layer 34 is then deposited on the smoothed dielectric material, and makes a continuous contact to the doped region 11 as indicated. Subsequent steps can be used to pattern the conducting region 34 to obtain a desired conductor pattern.
One consequence of this smoothing technique is that it increases the distance required for the top of the contact, referred to as the "head" of the contact. As shown in FIG. 3, this distance d' may be substantially greater than the distance d that initially defined the window. The increase of d' as compared to d then limits the density with which adjacent devices can be packed into a given area. Frequently, the distance d is defined by the minimum size that the lithographic technique utilized can achieve. In some cases, it is desirable to form the contact such that this minimum geometry is preserved. One technique that avoids flowing the p-glass layer is by depositing low-resistivity polysilicon into the windows prior to aluminum deposition. The polysilicon then provides electrical conductivity even if the aluminum is discontinuous; see U.S. Pat. No. 4,291,322 coassigned with the present invention. However, in some cases even lower electrical contact resistance is desired. In other cases, it is desirable to avoid forming doped (low resistivity) polysilicon, especially as in CMOS circuitry, when two different dopant types (p-type and n-type) are required In other instances, it has been found that the aluminum coverage is typically not as good as is desired, even after flowing the p-glass material. That is, small gaps or discontinuities can still exist in aluminum layer 34 deposited on flowed dielectric 12 in FIG. 3. It is desirable to have an alternate technique for making contact to an underlying layer that is separated from a conductive layer by a dielectric region.